1. Field of Art
Magnesium oxychloride cement has been used as a bonding agent since early in the twentieth century. While processes for preparing magnesium oxychloride cement bonds are well-known in the art, the chemistry of these processes and products is not fully understood to this day. The cement is a complex reaction product of calcined magnesium oxide, magnesium chloride, and water, which is admixed in conventional proportions and allowed to cure or harden into a cement variously designated in the industry as "magnesite cement", "magnetic cement", "Sorel cement", "French cement", and other terms.
Curable magnesium oxychloride cement compositions broadly range from castable mixes to mixes that are quite stiff and are tamped into molds. While the invention described herein is applicable to compositions encompassing at least this broad range of formulations, it is particularly applicable to the latter compositions, those that are stiff, and tamped or otherwise compacted into a mold such as a wheel mold, wherein they are cured to provide the desired shaped product.
Magnesium oxychloride cement has particular application in the production of abrasive tools used for grinding, especially abrasive wheels. These wheels are typically prepared as known in the art by admixing the bond precursors with abrasive grit and optional additives, molding the mixture into a wheel shape by the above-described tamping operation, and curing the mixture to harden the bond components into a magnesium oxychloride cement bond containing abrasive grit material of a type and amount suitable for the contemplated grinding application.
The product has grinding characteristics eminently suitable for dry grinding applications. Unfortunately, the magnesium oxychloride cement bond is quite brittle and has a low coefficient of thermal expansion, and the strength of the magnesite bond can rapidly deteriorate under the heat generated during dry grinding. The cause of this is not clear; however, dehydration of the oxychloride bond may be at least partially responsible.
Loss of heat strength may lead to cracking of the bonding cement. Abrasive magnesium oxychloride cement wheels are large and very heavy, and cracking of these wheels can result in premature failure of the wheel. Further, heat build-up during dry grinding contributes to metallurgical changes in metal substrate materials such as coil springs which, if sufficiently profound, result in visible burning of the product and an unacceptable product quality.
2. Discussion of Related Art
Traditionally, liquid coolants, often fortified with lubricant material, are employed to dissipate heat engendered in various grinding processes to protect both the abrasive tool and brittle or otherwise susceptible substrates from heat damage. Wet grinding applications employing liquid coolants are not, however, suitable for use in conjunction with magnesium oxychloride cement-bonded abrasive tools. Liquid coolants of the type commonly used in conjunction with wet grinding operations to dissipate heat build-up during grinding nearly always contain water, which softens and dissolves the magnesium oxychloride bond and causes rapid deterioration of the tool. Further, certain industrially-important substrates, notably the above-mentioned coil springs, do not lend themselves to wet grinding processes, which promote rusting of metal workpieces and impaction of swarf in the workpiece. Accordingly, such substrates are commonly dry-ground. As a result, magnesium oxychloride cement-bonded abrasive wheels or other abrasive tools are used almost exclusively for dry grinding applications, particularly for dry-grinding of water-sensitive substrate material such as coil springs. As discussed above, a rapid loss of bond strength typically accompanies the heat build-up characteristic of dry-grinding, and such abrasive tools are at risk of developing deep cracks under the heat generated during grinding, which may be followed by disintegration of the tool.
The incorporation of lubricants such as graphite into grinding tools is well-known. For example, U.S. patents U.S. Pat. No. 1,483,854 to Stern; U.S. Pat. No. 1,504,697 to Menard; U.S. Pat. No. 3,062,633 to Coes, Jr.; U.S. Pat. No. 3,718,447 to Hibbs, Jr. et al.; U.S. Pat. No. 3,868,233 to Carver, et al.; U.S. Pat. No. 3,779,727 to Siqui, et al.; U.S. Pat. No. 4,157,897 to Keat; U.S. Pat. No. 4,308,035 to Danilova et al.; U.S. Pat. No. 4,334,895 to Keat; U.S. Pat. No. 4,378,233 to Carver; and U.S. Pat. No. 5,030,282 to Matsuhashi et al.; all provide exemplary teachings of graphite or other lubricant in various grinding tool compositions. Stern; Coes, Jr: Hibbs, Jr; Siqui, et al.; Carver, et al.; and Danilova et al. describe abrasive materials comprising organic resin binders and solid lubricants such as graphite. Danilova, et al. disclose an abrasive material comprising mineral and organic binders and an antifriction material. The Keat and Carver patents are variously directed to ceramic-, glass-, or metal-bonded abrasive tools containing a dry film lubricant such as graphite. However, none of these references suggests the use of graphite in combination with magnesium oxychloride cement bonds.
It has been known for the decades that magnesium oxychloride cement has been in use as a bonding agent for abrasives in grinding applications that this bond is highly sensitive to the incorporation of extraneous materials, which in additive-effective amounts often weaken bonding of the cement precursors to the point of providing a commercially useless product. As noted above, the chemistry of the magnesium oxychloride cement bond is not clearly understood, and addition of extraneous materials to the basic combination of calcined magnesium oxychloride, magnesium chloride, and water with a view toward improving the formulation has of necessity proceeded on an ad hoc basis. Many additives have been suggested for improvement of grinding tools based on bonding agents such as Portland cements, vitrified bonds, or resinous materials such as phenol formaldehyde-based resins to promote structural strength or other characteristics of the product grinding tools. However, in combination with the present magnesium oxychloride cement precursors, such additives in effective amounts have frequently proved to be deleterious to the magnesium oxychloride bond, often weakening the bond structure of the product to the point of uselessness for the intended grinding application.